During the vegetative cycle you can prune and trim as much as you want. When the plant is flowering, it is best to leave it alone. You should only pull off the leaves the are yellow and dieing. 2 days before harvest it is best to stress the plant a little bit though. It will help a little bit with the resin production. I stop watering the plant and pick off the bigger fan leaves 2 days before harvest. This also makes your drying and curing process smoother.